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Could someone summarize PVST+, RPVST+ etc.

Reading through all this, it's a lot to take in, what do I need to know for the most part. There are so many with similar names, it's hard to differentiate. Could someone summarize them all with details that are mostly essential?


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Re: Could someone summarize PVST+, RPVST+ etc.


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Well first you need to know, if you're going to ask about STPs, the LAN forum would probably be better.

Next thing to know, STP is needed to break L2 loops.  If you don't have any L2 loops, you don't need to run a STP but it's often a good idea to do so, in case an accidental L2 loop is created.

On Cisco equipment, if equipment supports it, first choice is usually "spanning-tree mode rapid-pvst".  This offers all/most the "best" STP features, and since its per VLAN, you can have different STP topologies per VLAN.  If you don't want to support a STP per VLAN, "spanning-tree mode mst" offers all/most the features of RPVST+, but allows you to reduce the number of STP instances (which could be useful if you have lots of VLANs and/or you don't need multiple different L2 STP topologies).

Cisco's PVST+ supports about all the features of rapid-STP, but you need to activate many via configuration options.  They are also proprietary.

So, to recap, without explaining why, your best choices are rapid-pvst or mst.  A basic configuration of the former is a bit simpler than the latter, the latter can be more efficient (if you don't need different L2 STP topologies).

Re: Could someone summarize PVST+, RPVST+ etc.



- You can have a separate spanning tree instance for each VLAN you have configured

- Basic 802.1D

- Has modifications to help convergence (UplinkFast, BackboneFast, and Portfast)

- There are three types of BPDUs (Bridge Protocol Data Units)

- Topology Change occurs when a put goes from blocking to forwarding, and when a port goes

into a forwarding state

- The switch that notices a Topology Change will send a Topology Change Notification BPDU up through it's Root Port, and will continue doing so unlill it receives a Topology Change Acknowledgement. This will happen all the way up tot he Root Switch, and then the Root Switch will mark the Topology Change bit in the Configuration BPDU, and switches will lower their MAC Address expiration timer to 15 seconds.

- UplinkFast allows an alternate port upstream to be immediately switched over to sub-second instead of taking 30 seconds to go from Listening to Learning

- BackboneFast helps with indirect failures, speeding up convergence by not waiting Max_Age (20 seconds by default), and then taking 30 seconds (Listening and Learning) to move the port into forwarding

- Configuration BPDUs are sent by the Root Switch every 2 seconds

Rapid PVST+


- Configuration BPDUs are originated from every switch every two seconds.

- Max_Age is now 6 seconds which is way lower than the 20 seconds with PVST+

- You have less states (Discarding, Learning, and Forwarding)

- You have faster convergence than regular PVST+ due to point-to-point links running a SYNC process.

- Basically the Designated Port will send down a Configuration BPDU with the Proposal bit set, the switch receiving the switch receiving this will block all of its non-edge designated ports, and will come to a common conclusion that this port needs to the root port. Once all this information has been synd. It will send a Configuration BPDU with the Agreement bit set. This will basically start a wave, all the way to the edge, allowing for super fast convergence.

- Features such as Uplinkfast, BackboneFast, and Portfast are automatically included

- The super fast convergence speed will only work on point-to-point links (Full Duplex)

- Topology changes with RSTP, only occurs if a port goes into a forwarding state. Also, the switch noticing this topology change will start the TC While timer (2 times hello timer) so 4 seconds by default, and will immediately flush the MAC Addresses associated with all non-edge designated ports, basically all ports included in the Spanning Tree topology minus ports leading to hosts.

You can really write several books on this subject, but that's a real brief cliff notes version.